This invention relates to an improved extractive distillation process for separation and recovery of butadiene from mixtures of close boiling hydrocarbons. More particularly, this invention is directed to an improvement on the extractive distillation process described in U.S. Pat. No. 3,772,158, which avoids the problems associated with the buildup of C.sub.5 hydrocarbons in the solvent phase of extractive distillation when said extractive distillation process is employed subsequent to conventional fractionation to recover butadiene from a C.sub.4 hydrocarbon stream containing minor amounts of C.sub.5 saturated and unsaturated hydrocarbons.
Conventional processes for the production of butadiene, an important starting material in the manufacture of synthetic rubbers, pharmaceuticals and the like, invariably include a process step or sequence of process steps wherein butadiene must be separated and recovered from a mixture of hydrocarbon components whose normal volalities are such that separation cannot readily be achieved by ordinary fractional distillation. In these conventional processes, the source of butadiene is typically a non-selective dehydrogenation or pyrolysis operation which yields butadiene in admixture with a variety of other saturated and unsaturated hydrocarbons including compounds of like carbon number and different degrees of unsaturation which are very difficult to separate one from another. According to industry practice, butadiene is suitably recovered and purified in these instances through the application of an extractive distillation technique which alters the relative volatilities of the hydrocarbon components in the mixture and thereby makes the separation by distillation possible. In a typical extractive distillation procedure, the distillation of the butadiene-containing hydrocarbon mixture is carried out in the presence of a polar solvent such as acetonitrile, acetone, furfural, dimethylformamide and their corresponding aqueous mixtures, which enhances the volatility of the less unsaturated components relative to the more unsaturated compounds. In such an extractive distillation process, compounds such as butanes and butenes exhibit higher volatilities and are recovered as overhead product whereas diolefins such as butadiene and higher acetylenes, being less volatile, are separated together with the polar solvent as the fat solvent, bottoms product from the extractive distillation zone. Depending on the extractive distillation process selected, the butadiene-containing fat solvent is then passed to one or more flashing and/or stripping zones operated at elevated temperatures and/or reduced pressure for separation and recovery of the purified butadiene.
One conventional extractive distillation process which is eminently suitable for the recovery of butadiene from mixtures of close boiling hydrocarbons, including butanes and butenes, is that described in U.S. Pat. No. 3,772,158 to Sarno. In this process, conventional extractive distillation is carried out in the presence of a polar solvent e.g. acetonitrile, and butadiene is initially recovered from the fat solvent bottoms product at low pressure as a butadiene-rich vapor by sequential flashing and stripping of the unflashed liquid product in a flash zone and a first stripping zone operated at pressures lower than that of the extractive distillation zone. The butadiene-rich, vapor products of low pressure flashing and stripping are then combined and compressed to a pressure higher than that of the extractive distillation zone and a portion of this compressed vapor is recycled to the bottom of the extractive distillation zone with the remainder of the vapor being passed to a second stripping zone wherein butadiene is recovered as an overhead product. The process described in the Sarno patent is especially advantageous in providing for comparatively low temperatures in the flashing and stripping zones of the process, thus minimizing or avoiding polymerization of diolefins and acetylenes, while at the same time reducing the heat energy, refrigeration and compression requirements of the system.
While this patented process has found substantial commercial acceptance, there are situations in which its application is less than optimum. One such situation where problems arise is when the hydrocarbon feedstock to extractive distillation contains minor but significant amounts of C.sub.5 saturated and/or unsaturated hydrocarbons. This situation typically occurs in the recovery of butadiene from naphtha pyrolysis effluents or refinery butenes where a hydrocarbon stream containing compounds of higher molecular weight than butadiene, including significant amounts of C.sub.5 hydrocarbons such as n-pentane, isopentane and 3-methyl butene-1, butene-1, is conventionally subject to a fractionation or distillation to remove the heavier hydrocarbons prior to extractive distillation. In this situation, misoperation, changes in the feed composition and/or inadequate design of the fractionation zone upstream from extractive distillation can result in significant amounts of C.sub.5 hydrocarbons being carried over with the butadiene-containing overhead from fractionation into the extractive distillation zone. In extractive distillation and subsequent butadiene recovery process stages involving flashing and stripping, these C.sub.5 hydrocarbons exhibit inconsistent behavior, acting as heavy hydrocarbons in a hydrocarbon environment while having the volatility characteristics of light hydrocarbons in solvent environment. The net effect is that it is very difficult to fractionate mixtures containing both C.sub.4 and C.sub.5 hydrocarbons in addition to polar solvent and as a result, C.sub.5 hydrocarbons tend to accumulate in the process flow subsequent to extractive distillation. The accumulation of these C.sub.5 hydrocarbons is also undesirable in that they have a deleterious effect on the rejection of other contaminants from the butadiene product, in particular 1,2-butadiene and vinyl acetylene, without also incurring excessive solvent loses.
Accordingly, it would be desireable if the extractive distillation process described in U.S. Pat. No. 3,772,158 could be modified in some fashion to eliminate the problems associated with the accumulation of C.sub.5 hydrocarbons in the processing steps subsequent to extractive distillation in cases where the hydrocarbon feedstock to extractive distillation is derived from an initial fractionation which unavoidably leaves a minor amount of C.sub.5 hydrocarbons in the feedstock. Further, it would be especially advantageous if the accumulation of C.sub.5 hydrocarbons could be eliminated or minimized in a way which does not materially increase the cost and complexity of the process.